I. Field of the Invention
This invention relates to a biaxially oriented poly-p-phenylene sulfide film. More particularly, this invention relates to a biaxially oriented poly-p-phenylene sulfide film with excellent mechanical properties and dimensional stability.
II. Description of the Related Art
In recent years, biaxially oriented films of poly-p-phenylene sulfide (hereinafter referred to as PPS for short) are drawing attention as F class electric insulating thin materials in the field of various electric apparatuses and electronic parts because of their excellent thermal resistance and hydrolysis resistance. Although conventional PPS films have a number of advantages, they also have drawbacks. when compared with polyethyleneterephthalate films which are widely used industrial films, in that they have poor mechanical properties, especially low tensile strength at break, and have too large thermal expansion coefficient.
PPS resins have been mainly studied as a resin for injection molding. Therefore, as to the resin composition suitable for forming PPS films, although there are teachings about the molecular weight of PPS and its degree of crosslinkage (or the viscosity of molten PPS and non-Newtonian coefficient) (Japanese Patent Disclosure (Kokai) No. 62121/81) and the relationship between the viscosity of molten PPS and its molecular weight (Japanese Patent Disclosure (Kokai) No. 89026/86), the idea to link the chemical composition of the polymer or the molecular weight distribution of the polymer with the film properties has not yet been suggested at all.
It is known, however, that a PPS resin suitable for forming films may be obtained by reducing the content of sodium chloride which is a byproduct as small as possible by intensively washing the polymer with water. However, by this method, although the content of the contaminants is decreased so that the clearness of the obtained film is improved, the mechanical properties and dimensional stability of the obtained film are not satisfactory.
Further, it is known that crystallization of PPS can be influenced by treating PPS with an oxide or a hydroxide of a metal cation so as to incorporate the metal cation in the polymer in the amount of 50-5000 ppm (Japanese Patent Disclosure (Kokai) No. 78257/82). By this method, although the rate of crystallization can be promoted, the mechanical properties and the dimensional stability of the resulting film are not satisfactory.